The document (Nature Vol. 277, pp 637-638, 1979) discloses a technique for producing formaldehyde, formic acid, methane, methanol, etc. by suspending a catalyst powder formed of a semiconductor such as titanium oxide in water and supplying carbon dioxide to the suspension solution while applying light from an artificial light source such as a xenon lamp or a high-pressure mercury-vapor lamp.
Furthermore, Japanese Patent No. 2526396 discloses a method for producing hydrogen and oxygen from water by use of optical energy; more specifically, by irradiating a zirconium oxide semiconductor with light, and a method for producing hydrogen, oxygen, and carbon monoxide from water and carbon dioxide.
Moreover, Japanese Patent No. 3590837 discloses a technique for selectively reducing carbon dioxide to carbon monoxide by dissolving a photocatalyst, which is selected from metal complexes whose charge absorption band between a metal and a ligand falls within the range from the ultraviolet light region to the visible light region, and an electron donor selected from organic amines; introducing carbon dioxide into the organic solvent at a high pressure of 0.2 to 7.5 MPa; and applying light to the organic solvent under the pressure.
In the document Nature (Vol. 277, pp 637-638, 1979) mentioned above, formaldehyde, formic acid, methane, methanol, etc. are produced simultaneously; whereas, in Japanese Patent No. 2526396, hydrogen alone is produced or hydrogen and carbon monoxide are produced simultaneously. An inorganic semiconductor photocatalyst is characterized in that hydrogen or not less than two types of reduced forms of carbon dioxide can be simultaneously produced by light irradiation. However, from an industrial point of view, the ability to obtain a product with high selectivity is important.
Furthermore, Japanese Patent No. 3590837 mentioned above shows reduction treatment using a rhenium complex. When a rhenium complex is used, it is known that carbon monoxide tends to be advantageously and selectively produced. When a rhenium complex is used a photocatalyst, the rhenium complex absorbs visible light (realizing a carbon dioxide reductive reaction) within the range of 450 nm or less. Absorption light is limited to a relatively short wavelength.
Japanese Patent No. 3590837 mentioned above describes other metal complexes; however, possible metal elements are simply enumerated, and they are not put into practice.
Further, although it is known in the field of a dye-sensitized solar cell that a ruthenium complex can absorb light having long wavelength depending on its Ru(bpy)3 (bpy: bipyridine) structure, photocatalytic reaction is not obtained with such a structure. The ruthenium complex Ru(bpy)2(CO)2 having this structure actually carries out an electrochemical catalytic reaction alone to produce formic acid from carbon dioxide with high product selectivity.